Hullo.
I had similar pinboard issues a year or two ago. You might be able to find the thread I started at the time, but eventually I gave up because someone who replied turned out to be rather perverse and unhelpful, so I gave up.
What I did learn is that pinboards are stored in the "bookmarks" file. On my ancient Win7 PC this is located at:
C:\Users\Playtech\AppData\Roaming\Opera Software\Opera Stable
This file seems to back itself up to bookmarks.bak in the same location, though my attempts to restore pinboards at that time were unsuccessful (like yourself, I'm just a button-pusher with no technical aptitude). However, both files are included in my daily backup routine.
The other thing I learned is that if Opera is set to delete cookies on exit it will also delete your pinboards. How stupid is that? Consequently i never delete cookies and they just accumulate—probably in the thousands now, and there's no way I can see to select which to delete and which to retain.
It's a crying shame that such a useful feature should be so vulnerable to unintended deletion without warning. Even after all this time pinboards are still shown as "beta", and it appears nothing has been or is being done to rectify the issue and make it more robust. This reminds me of the old English proverb about "spoiling the ship for a ha'porth of tar".
Sorry I can't be of more help. It would be good if someone who really understands these issues would contribute to the discussion.